The present application claims priority from Japanese Patent Application No. 2014-206749, filed Oct. 7, 2014, and Japanese Patent Application No. 2014-206750, filed Oct. 7, 2014, which are incorporated herein by reference.
The present invention generally relates to a two-stroke internal-combustion engine and more specifically relates to an air leading-type engine that first induces air to flow into a combustion chamber in a scavenging stroke.
Two-stroke internal-combustion engines are often used in portable work machines such as brush cutters and chain saws. This type of two-stroke internal-combustion engine includes a scavenging channel that brings a crankcase and a combustion chamber into communication with each other. Air-fuel mixture pre-compressed in the crankcase is induced to flow into the combustion chamber through the scavenging channel, and scavenging is performed by the air-fuel mixture.
As well-known, two-stroke engines of the type in which scavenging is performed using air-fuel mixture have the problem of “air-fuel mixture (new gas) blow-by”. In response to this problem, air leading-type stratified scavenging two-stroke internal-combustion engines have been proposed and already put into practical use. See U.S. Pat. No. 6,857,402, for example. Prior to scavenging, the air leading-type stratified scavenging engine charges air to a scavenging channel. In a scavenging stroke, first, the air in the scavenging channel is discharged to the combustion chamber, and then the air-fuel mixture in the crankcase is induced to flow into the combustion chamber through the scavenging channel.
FIG. 14 is a diagram illustrating a conventional air leading-type stratified scavenging engine. In FIG. 14, in order to avoid confusion of drawn lines, illustration of a piston is omitted. In the figure, reference numeral 100 denotes a cylinder wall. In the cylinder wall 100, an air channel 102 and an air-fuel mixture channel (not shown) open. An air port is indicated by reference numeral 102a. Also, in the cylinder wall 100, a scavenging port 104a of a scavenging channel 104 opens. The scavenging channel 104 communicates with a crankcase. Each of the air port 102a and the scavenging port 104a is opened/closed by the piston. The piston has a groove 106 in a peripheral surface thereof. The piston groove 106 extends in a circumferential direction.
(I) to (III) of FIG. 14 indicate states in the course of a piston moving up: (II) of FIG. 14 indicates a state in which the piston moves up relative to the position in (I) of FIG. 14. (III) of FIG. 14 indicates a state in which the piston moves up relative to the position in (II) of FIG. 14.
Referring to (I) of FIG. 14, in the piston groove 106, a gas blown back in previous scavenging process is mixed. The blown-back gas contains air-fuel mixture components. The blown-back gas remaining in the piston groove 106 is indicated by dots. Along with upward movement of the piston from the bottom dead center, a pressure in the crankcase becomes negative. (II) of FIG. 14 illustrates a state in which the piston groove 106 communicates with the air port 102a. In the state in (II) of FIG. 14, the piston groove 106 is not in communication with the scavenging port 104a. Therefore, even though the piston groove 106 communicates with the air port 102a, no air flows from the air port 102a into the piston groove 106. In other words, the blown-back gas in the piston groove 106 does not flow.
(III) in FIG. 14 indicates a state in which the piston groove 106 communicates the air port 102a and also communicates with the scavenging port 104a. As a result of the piston groove 106 coming into communication with the scavenging port 104a, air can be supplied from the air port 102a to the scavenging channel 104 via the piston groove 106.
With reference to (III) in FIG. 14, in theory, in a conventional air leading-type stratified scavenging two-stroke internal-combustion engine, a flow of gas in the piston groove 106 occurs only when the piston groove 106 communicates with the scavenging port 104a. Then, the gas in the piston groove 106 first enters the scavenging channel 104, and then air enters from the air port 102a to the scavenging channel 104 through the piston groove 106. Therefore, a timing of the air entering the scavenging channel 104 from the piston groove 106 is later than a timing of the piston groove 106 starting communicating with the scavenging channel 104.
As well-known, for air leading-type two-stroke internal-combustion engines for work machines, piston valve-type ones are employed. In other words, an air port 102a, a scavenging port 104a, and an exhaust port and the like are opened/closed by a piston. In a piston valve-type engine, a gas flow is controlled by a pressure balance between two spaces or channels that communicate with each other or are isolated from each other via a piston.
A two-stroke engine for a work machine is run at a high rotation rate of, for example, 10,000 rpm. Therefore, the aforementioned timing delay largely affects the efficiency of air charge into a scavenging channel 104. In other words, conventional stratified scavenging two-stroke engines have the essential problem of difficulty in ensuring the certainty of charging air into the scavenging channel 104 in each cycle.
In a scavenging stroke, an air leading-type stratified scavenging engine first discharges burned gas by means of air and then charges air-fuel mixture into a combustion chamber. In theory, employment of the air leading-type stratified scavenging method should enable substantial improvement in emission characteristics. However, in reality, the emission characteristics improvement effect is limited by the aforementioned essential problem.
In order to respond to the aforementioned timing delay, substantially advancing a timing for the piston groove 106 to communicate with the scavenging port 104a has been proposed. However, employment of this configuration results in the air-fuel mixture components remaining in the scavenging channel 104 easily flowing to the air channel 102 side, which causes decrease in emission characteristic improvement effect.
An object of the present invention is to provide an air leading-type stratified scavenging two-stroke internal-combustion engine that can improve the certainty of supplying air to a scavenging channel through a piston groove.
Another object of the present invention is to provide an air leading-type stratified scavenging two-stroke internal-combustion engine that can improve the certainty of an amount of air to be supplied to a scavenging channel through a piston groove.
A still another object of the present invention is to provide an air leading-type stratified scavenging two-stroke internal-combustion engine that can improve the certainty of an air supply timing for supplying air to a scavenging channel through a piston groove.